1. Field of Invention
The present invention relates to a type of lead frame used in the packaging of an IC chip. More particularly, the present invention relates to a lead frame with an additional heat slug that can serve as a radiating surface outside the IC package. In addition, the heat slug is able to hide burrs outside the package so that there is less problem in transporting the package.
2. Description of Related Art
In the manufacturing of semiconductor IC, packaging is the last but also one of the more critical steps in producing a reliable product. In IC packaging, a fully functional IC chip is mounted onto an IC lead frame. Then, the IC chip is electrically connected to the internal lead section of the lead frame. After connection to the internal leads, the chip and internal wiring are sealed, the external leads of a complete IC package can be electrically connected to appropriate terminals on a printed circuit board (PCB). Hence, the IC package and the PCB are combined into one big operating circuit.
FIG. 1 is a schematic, perspective view of an IC package using a conventional lead frame. FIG. 2 is a schematic, top view showing the internal layout of a conventional lead frame. As shown in FIGS. 1 and 2, a lead frame has a sheet-like structure that can be roughly divided into a planar region and leads region. The planar region is usually referred to as a bar pad or a die pad 10. The die pad 10 is used for mounting a silicon chip. The leads region is often subdivided further into an internal lead section 15 and an external lead section 25. Alternatively, a lead frame can be divided up into a packaging area 50 (area enclosed within centered lines) and an external area. In fact, the package area 50 is the place in which the silicon chip, the wiring and lead frame are eventually sealed off. To increase the degree of automation, various lead frames are placed side by side using with the help of two side rails 30.
The conventional method of packaging an IC chip includes placing a silicon chip onto a die pad 10, and then fixing the chip in place with die attach material. Next, the bonding pads on a silicon chip are connected to the respective leads by metallic wires through a wire bonding operation. Most often gold or aluminum wires are used. Thereafter, the lead frame is placed inside a set of molds, and the packaging area 50 is sealed off using epoxy molding compound. Finally, the external portion of the leads are trimmed and then formed into the desired shape, thereby producing a complete IC package.
As shown in FIG. 2, end flashes come out between the leads at the boundary of the packaging area 50. Even after the side rails 30 are cut off, traces of these flashes can still be seen. These end flashes are often referred to as burrs 80 (FIG. 1). As long as the packaging area 50 of a lead frame is sealed by injection molding material into a mold cavity, some of the injected material is bound to leak out from the junction between the molds and the lead frame. Hence, these burrs are unavoidable. Although some of these burrs can be removed in a subsequent operation, the removal process adds to the cost of production.
Moreover, some burrs remain on the package in spite of the use of high-precision deburring tools. After IC packages are separated from each other, each IC unit is individually transported to an automatic testing through a vibrator, and then is carried on along a track to a packaging station. In general, the IC packages line up close to each other along the track. However, burrs 80 outside the package may increase friction, so that when they rub against each other and against the tracks, some of the packages may be displaced, thereby jamming the tracks. Hence, a lot of production time is wasted simply to clear the jammed tracks.
Another genuine problem of IC package is the vast amount of heat generated when a silicon chip inside the package is operating a full capacity. The heating problem is becoming more serious, especially as the level of integration on a semiconductor substrate continues to increase and the processing power of a silicon chip continues to multiply.
As shown in FIG. 2, the entire packaging area 50 is enclosed. Therefore, any heat generated internally has to pass out by heat conduction through the packaging material. Since most packaging material is generally a good insulator or a poor heat conductor, heat conduction particularly difficult. To boost of the amount of heat dissipated through an IC package, sometimes a sheet of radiating metal is attached to the lead frame before sealing. Hence, heat generated by the silicon chip can be conducted away through the radiating surface of the metal sheet. However, adding a radiator not only increases the complexity of the packaging operation, but both material cost and production cost increase as well.
In light of the foregoing, there is a need to provide a better lead frame design that can remove more heat and prevent the resulting semiconductor package from jamming tracks.